Conventionally, to continuously supply a power source to a load such as electric equipment typified by an artificial heart system or the like, a plurality of power sources are connected to a power source switching circuit, and the power source switching circuit is configured to select one power source. The power source switching circuit of this type includes, for example, a field effect transistor (hereinafter referred to as “FET”) which constitutes an electronic switch. The power source switching circuit switches a supply source of power source by controlling turning on or off of the FET using a logic circuit or a microcomputer. Techniques relating to such a power source switching circuit are disclosed in patent document 1 to patent document 5, for example.
Patent document 1 discloses the technique where an erroneous operation of a PMOS transistor can be prevented in a backup operation by preventing the generation of a potential difference between a gate voltage and a source voltage of the PMOS transistor connected between a main power source and a sub power source. Patent document 2 discloses the technique where at the time of switching from one power source to the other power source, an output of one power source is stopped in response to an output of the other power source thus preventing an instantaneous drop of a supply voltage. Patent document 3 discloses the technique where a voltage of a main power source and a reference voltage are compared to each other, and even if the main power source is selected when the voltage of the main power source becomes lower than a voltage of a backup power source, an electric current does not flow into a load from the backup power source. Patent document 4 discloses the technique of switching power sources using relays in the constitution where the diode OR connection is adopted. Patent document 5 discloses the technique where a means for the determination of the occurrence of a failure is provided, and when a failure occurs in one system, it is possible to prevent the failure from affecting other systems.